The program concerns mechanisms of calcification in molluscs and marine algae, which provide unusually favorable experimental material for calcification studies. Cobalt has been found to cause reversible inhibition of cell division in the alga Cricosphaera. The studies are being extended to determine the effects of cobalt on division of HeLa cells and on calcification of chick bone growth. Ultrastructural changes in Cricosphaera produced by cobalt are being examined to determine their time course and reversibility. The snail Otala provides a model system for following polymerization of scleroproteins of the organic matrix of shell as it calcifies. The ability of various stages of polymerization of the organic matrix to induce calcification in vitro is being studied.